In cementing casing in well bores of subsea oil and gas wells, it is common industry practice to employ a cement plug launching device near the top of the well bore casing, which may be as much as several thousand feet below the water surface, rather than launching the plugs from the floor of the offshore platform. There are several major reasons for launching plugs remotely, through drill pipe extending from the platform to an installation tool at or near the top of the well bore casing, even though it is difficult if not impossible to ascertain from the rig floor on the platform if the launching device has operated properly. First, well casing is of relatively thin wall construction and large diameter, with threaded connections designed for permanent installation in a well bore, rather than ease of assembly and disassembly. Therefore, running casing from the platform to the sea floor is difficult and expensive. In addition, after the well is cemented, the casing run between the platform and the sea floor would have to be retrieved, and returned to shore, there being no further use therefor. Of course, there is also the initial problem of transporting casing to the platform in the first instance. Finally, once casing has been used, many well operators will not permit subsequent reuse on another well for safety and reliability reasons; therefore the cost of casing for use in one well becomes prohibitive. Using a remote plug launching device actuated via drill pipe from the platform to the device instead of casing, affords several major advantages: drill pipe is of relatively small diameter and of sturdy construction, permitting ease of handling and greater safety; drill pipe has threads machined for rapid and repeated connection and disconnection; the use of drill pipe permits inclusion of telescoping slip joints or bumper subs in the pipe string in order to compensate for the vertical motion of the platform imparted by wave action.
The major disadvantage of employing a remote cement plug launching device is, as previously noted, the possibility that the device will operate improperly without detection by the well operator. For example, the launching device may prematurely release one or more of its plugs, or fail to release a plug at all.
One remote plug launching device designed to avoid these operability problems is disclosed in U.S. Pat. No. 3,915,226 to Ronald E. Savage, assigned to the assignee of the present application, and incorporated herein by reference. This prior art device comprises a double-collet type release mechanism which ensures reliable release of the top, or cement displacement plug from the drill pipe. However, the release of the bottom, or fluid displacement, plug is effected by using a free-fall ball which is placed in the drill pipe at the surface, and allowed to fall to the bottom plug, wherein it seats, and subsequent application of pressure in the drill pipe shears pins holding the bottom plug to the top plug, releasing the former. This use of a free-fall type ball, however, presents a problem in deeper offshore wells where the volumetric capacity of the drill pipe is greater than the volume of spacer fluid run ahead of the cement pumped thereafter, due to the uncertainty as to when the ball will reach the bottom plug, the tendency of the spacer fluid to incompletely displace the drilling and below it in the well, and the inability of the ball to wipe the inner wall of the drill pipe clean of mud, all of which result in the presence of mud above the bottom plug when it is released, with consequential mud contamination of the cement following the bottom plug.
A number of other prior art plug launching devices suffer from the same enumerated disadvantages, including those disclosed in U.S. Pat. Nos. RE 29,830, 3,616,850, 3,730,267, 4,042,014, 4,047,566 and 4,164,980.
One solution to the aforementioned use of a freefall ball to release a bottom plug is suggested in U.S. Pat. No. 3,796,260, issued to B. Jack Bradley, assigned to the assignee of the present invention, and incorporated herein by reference. The Bradley patent discloses the use of drill pipe plugs or darts which are pumped down the drill pipe to the cementing plugs, the cementing plugs having differently sized seats therein, with the lowermost cementing plug having the smallest seat. With such a design, a dart sized to move through the seats one or more cementing plugs above the bottom plug will seat on the bottom plug, and shear a pin, which releases the bottom plug. Thereafter, when desired, the next-larger dart is pumped down the drill pipe to seat in the lowermost remaining plug, and so on.
While a device such as is disclosed in the Bradley patent presents no problems with cementing plugs employed in small-size casing (under approximately seven (7) inches internal diameter), with larger casing the operator cannot be sure that the darts will consistently enter the seats in the centers of the cementing plugs and effect release of the plugs.